68 THE SECOND DAY. [CHAP. 



forming immediately behind'the heart the omphalo-mesaraic veins (section J?), (of). 

 As however the folding of the splanchnopleure becomes more complete, and the 

 digestive canal becomes completely closed (instead of remaining partially open 

 as in section B), these two cavities unite; and an appearance is produced 

 similar to that represented in figure A, where there is the single cavity of the 

 heart (hz). In the interior of the heart is seen a lining of flattened cells. 



The shading, as will be seen, is purely diagrammatic. The epiblast, 

 whether superficial as at E, or involuted as part of the neural canal hb, is 

 shaded of one tint. The mesoblast, whether uncleft, or diverging into soma- 

 topleure and splanchnopleure, is of another tint. In the hypoblast a distinction 

 has been drawn between the thickened portion which lines the alimentary 

 canal, and the thinner portion which belongs to the more peripheral part 

 of the splanchnopleure, the two being at first continuous as in B, and afterwards 

 separated as in A. 



It will be understood that the two figures, though actually two consecutive 

 sections of the same embryo, may be taken to represent two phases of the 

 formation of the heart. B in the process of development will become A, and 

 A a short time previously was in the condition of B. 



TRANSVERSE SECTION OF AN EMBRYO AT THE END OF THE SECOND DAY PASSING 



THROUGH THE REGION OF BULBUS ARTEttlOSUS. (Copied from His.) 



M. medullary canal in the region of the hind brain. V. anterior cardinal or 

 superior vertebral vein. Ao. Aorta. Ch. Notochord. al. alimentary 

 canal. H. Heart (bulbus arteriosus). Pp. Pleuroperitoneal cavity. 

 am. amnion. 



On comparing this with Fig. 17, it will be seen that the mesoblast (muscular) 

 wall of the heart H has now become quite separate from the rest of the 

 mesoblast of the splanchnopleure, which forms, in the section, an independent 

 line below the heart, the section of branches of the omphalo-mesaraic veins 

 being seen on either side. The bridle of mesoblast represented in the drawing as 

 passing from the splanchnopleure below to the sounatopleure above, reaching the 

 latter just inside the fold of the anmion, has been described by His, but has 

 never been seen by ourselves. 



In the vascular and pellucid areas, the formation of 

 vascular channels with a subsequent differentiation into 

 arteries, capillaries and veins, is proceeding rapidly. Blood- 



